This invention is, as indicated, an improvement on a combined electrically heatable converter including an electrically heatable portion and a "light-off" portion, each portion being made up of corrugated thin metal strips or layers in alternating relation with flat thin metal strips or layers. Electrical power is supplied to the electrically heatable portion only. Such devices are described in co-pending, commonly owned U.S. patent applications Ser. No. 08/013,516 filed 3 Feb. 1993 by Whittenberger and Woodruff, and 08/066,887 filed 25 May 1993 by Whittenberger. These applications are incorporated herein by reference thereto.
The invention will be described in connection with embodiments especially adapted for use in exhaust lines from various types of engines, e.g., internal combustion engines of the spark ignited or compression ignited types, stationary or mobile, or gas turbine engines. It will be understood, however, that the converters of the present invention may be used to effect various chemical reactions, particularly those occurring in fluid streams, especially gas streams, which reactions are catalyzed or uncatalyzed.
Turning now to converters especially useful in exhaust lines extending from internal combustion engines, e.g., those used in automotive vehicles, the purpose of such catalytic converters is to convert pollutant materials present in the exhaust stream, e.g., carbon monoxide, unburned hydrocarbons, ozone, nitrogen oxides, etc., to carbon dioxide, nitrogen oxygen and water prior to discharge to the atmosphere. Conventional automotive catalytic converters utilize an oval or circular cross-section ceramic honeycomb monolith having square or triangular straight-through openings or cells with catalyst deposited on the walls of the cells; catalyst coated refractory metal oxide beads or pellets, e.g., alumina beads; or a corrugated thin metal foil multicelled honeycomb monolith having a refractory metal oxide coating and catalyst carried on the coating and supported on the surfaces of the cells. The catalyst is normally a noble metal, e.g., platinum, palladium, rhodium, ruthenium, or a mixture of two or more of such metals. Zeolite coatings may also be used for the adsorption and desorption of pollutants to aid in their removal. The catalyst catalyzes a chemical reaction, mainly oxidation, whereby the pollutant is converted to a harmless by-product which passes through the exhaust system to the atmosphere.
However, conversion to such harmless by-products is not efficient initially when the exhaust gases are relatively cold, e.g., at cold engine start. To be effective at a high conversion rate, the catalyst and the surface of the converter must be at or above a minimum temperature, e.g., 390 degrees F. for carbon monoxide, 570 degrees F. for volatile organic compounds, and about 900 degrees F. for methane or natural gas. Otherwise, conversion to harmless by-products is poor and cold start pollution of the atmosphere is high. It has been estimated that as much as 80% of the atmospheric pollution caused by vehicles even though equipped with conventional non-electrically heated catalytic converters, occurs in the first two minutes of operation of the engine from cold start. Once the exhaust system has reached its normal operating temperature, a non-electrically heated catalytic converter is optimally effective. Hence, it is necessary for the relatively cold exhaust gases to make contact with hot catalyst so as to effect satisfactory conversion. Compression ignited engines, spark ignited engines, reactors in gas turbines, small bore engines such as used in lawn mowers, trimmers, boat engines, and the like have this need.
To achieve initial heating of the catalyst at engine start-up, there is conveniently provided an electrically heatable catalytic converter unit, preferably one formed of a thin metal honeycomb monolith. The monolith may be formed of spaced flat thin metal strips, straight-through corrugated thin metal strips, pattern corrugated thin metal strips, e.g., herringbone or chevron corrugated thin metal strips, or variable pitch corrugated thin metal strips (such as disclosed in U.S. Pat. No. 4,810,588 dated 7 Mar. 1989 to Bullock et al) or a combination thereof, which monolith is connected to a 12 volt to 108 volt or higher AC or DC voltage source, single or multi-phase, preferably at the time of engine start-up and afterwards to elevate the catalyst to and maintain the catalyst at a temperature of at least 650 degrees F. plus or minus 30 degrees F. Alternatively, power may also be supplied for a few seconds prior to engine start-up.
Catalytic converters containing a corrugated thin metal (stainless steel) monolith have been known since at least the early 1970's. See Kitzner U.S. Pat. Nos. 3,768,982 and 3,770,389 each dated 30 Oct. 1973. More recently, corrugated thin metal monoliths have been disclosed in U.S. Pat. No. 4,711,009 dated 8 Dec. 1987 to Cornelison et al; U.S. Pat. Nos. 4,152,302 dated 1 May 1979, 4,273,681 dated 16 Jun. 1981, 4,282,186 dated 4 Aug. 1981, 4,381,590 dated 3 May 1983, 4,400,860 dated 30 Aug. 1983, 4,519,120, dated 28 May 1985, 4,521,947 dated 11 Jun. 1985, 4,647,435 dated 3 Mar. 1987, 4,665,051, dated 12 May 1987 all to Nonnenmann alone or with another; U.S. Pat. No. 5,070,694 dated 10 Dec. 1991 to Whittenberger; International PCT Publication Numbers WO 89/10470 (EP 412,086) and WO 89/10471 (EP 412,103) each filed 2 Nov. 1989 claiming priority date of 25 Apr. 1988. The above International Publication Numbers disclose methods and apparatus for increasing the internal resistance of the device by placing spaced discs in series, or electrically insulating intermediate layers. Another International PCT Publication is WO 90/12951 published 9 Apr. 1990 and claiming a priority date of 21 Apr. 1989, seeks to improve axial strength by form locking layers of insulated plates. Another reference which seeks to improve axial strength is U.S. Pat. No. 5,055,275 dated 8 Oct. 1991 to Kannainian et al. Reference may also be had to PCT Publication Number WO 92/13636 filed 29 Jan. 1992 claiming a priority date of 31 Jan. 1991. This application relates to a honeycomb body along an axis of which fluid can flow through a plurality of channels. The honeycomb has at least two discs in spaced relation to each other. According to the specification, there is at least one bar type support near the axis by which the discs are connected together and mutually supported. The invention is said to make possible design of the first disc for fast heating up through through exhaust gas passing through or applied electrical current. The honeycomb body serves as a bearer for catalyst in the exhaust system of an internal combustion engine. Another reference is German Patent Application Number 4,102,890 A1 filed 31 Jan. 1991 and published 6 Aug. 1992. This application discloses a spirally wound corrugated and flat strip combination wherein the flat strip contains slots and perforations and is electrically heatable. The flat strips include a bridge between leading and trailing portions. Groups of such strips are separated by insulation means. The core is provided with a pair of circular retainer segments which are separated by insulation means. Another reference is U.S. Pat. No. 5,102,743 dated 7 Apr. 1992 to Maus. This patent discloses a honeycomb catalyst carrier body of round, oval or elliptical cross-section including a jacket tube and a stack of at least partially structured sheet metal alyers intertwined in different directions in the jacket tube. The stack has a given length and a given width. At least one of the sheet metal layers has a greater thickness over at least part of one of the dimensions than others of the layers. Such at least one layer is formed of thicker metal or of a plurality of identically structured metal sheets in contiguous relation.
Most recently, combined electrically heatable and "light-off" converters have been provided in which electrically heatable flat thin metal strips are alternated with non-electrically heatable corrugated thin metal strips and spirally wound in a jacket tube to provide what is called an integral structure, that is, where the electrically heatable portion is tied to the "light-off" portion by interleafing the corrugated thin metal sheets with the flat, electrically heatable thin metal sheets, and intertwining the assembly, as by spirally winding, into a suitable housing or jacket tube. Such structures are disclosed in the aforesaid Ser. No. 08/013,516 and 08/066,887.
A common problem with prior devices has been their inability to survive severe automotive industry durability tests which are known as the Hot Shake Test and the Hot Cycling Test.
The Hot Shake Test involves oscillating (100 to 200 Hertz and 28 to 60 G inertial loading) the device in a vertical attitude at a high temperature (between 800 and 950 degrees C.; 1472 to 1742 degrees F., respectively) with exhaust gas from a running internal combustion engine simultaneously passing through the device. If the catalytic device telescopes or displays separation or folding over of the leading or upstream edges of the foil leaves up to a predetermined time, e.g., 5 to 200 hours, the device is said to fail the test. Five hours is equivalent to 1.8 million cycles at 100 Hertz.
The Hot Cycling Test is one with exhaust gas flowing at 800 to 950 degrees C.; 1472 to 1742 degrees F.) and cycled to 120 to 150 degrees C. once every 15 to 20 minutes, for 300 hours. Telescoping or separation of the leading edges of the thin metal foil strips is considered a failure.
The Hot Shake Test and the Hot Cycling Test are hereinafter called "Hot Tests" and have proved very difficult to survive. Other durability tests are known for EHC's including an electrical cycling test. The EHC is heated to 500-700 degrees C. at rated power and then fan cooled, cycling every 1-3 minutes. This is done for 50,000 cycles. There should be no changes in electrical resistance.
The structures of the present invention will survive these Hot Tests.
In the following description, reference will be made to "ferritic" stainless steel. A suitable ferritic stainless steel for use particularly in the engine exhaust applications hereof is described in U.S. Pat. No. 4,414,023 dated 8 Nov. 1983 to Aggen. A specific ferritic stainless steel alloy useful herein contains 20% chromium, 5% aluminum, and from 0.002% to 0.05% of at least one rate earth metal selected from cerium, lanthanum, neodymium, yttrium, and praseodymium, or a mixture of two or more of such rare earth metals, balance iron and trace steel making impurities. A ferritic stainless steel is commercially available from Allegheny Ludlum Steel Co. under the trademark "Alfa IV." Another metal alloy especially useful herein is identified as Haynes 214 alloy which is commercially available. This alloy and other nickeliferous alloys are described in U.S. Pat. No. 4,671,931 date 9 Jun. 1987 to Herchenroeder et al. A specific example contains 75% nickel, 16% chromium; 4.5% aluminum, 3% iron, optionally trace amounts of one or more rare earth metals except yttrium, 0.05% carbon and steel making impurities. Haynes 230 alloy, also useful herein, has a composition containing 22% chromium, 14% tungsten, 2% molybdenum, 0.10% carbon, and a trace amount of lanthanum, balance nickel. The ferritic stainless steels and the Haynes alloys 214 and 230 are examples of high temperature resistive, oxidation resistant (or corrosion resistant) metal alloys that are suitable for use in making thin metal strips for use in the converter bodies hereof, and particularly for making heater strips for the EHC (electrically heatable converter) portions and "light-off" portions hereof. Suitable metals must be able to withstand "high" temperatures of 900 degrees C. to 1200 degrees C. (1652 degrees F. to 2012 degrees F.) over prolonged periods.
Other high temperature resistive, oxidation resistant metals are known and may be used herein. For most applications, and particularly automotive applications, these alloys are used a "thin" metal strips, that is, having a thickness of from about 0.001" to about 0.005", and preferably from 0.0015" to about 0.003".